Field of the Invention
This invention relates to an apparatus with a sealing. In the following, the invention will by way of example be explained by referring to an apparatus for heating a glass preform which is used for drawing an optical fiber, though it should be observed that the invention may also be utilized for other purposes.
Description of Prior Art
Previously a furnace is known for heating a glass preform such that the glass preform may be drawn into an optical fiber. This known furnace comprises a vertical center hole with surrounding heating elements. The glass preform is fed into this furnace from above, and while the glass preform is heated, an optical fiber may be drawn from a softened lower end of the glass preform.
In order to ensure that the properties of the drawn fiber are excellent, it is necessary to seal off the interior of the furnace from the surrounding environment. In the previously known furnace, a graphite ring is arranged at the top end of the furnace as a sealing. In this solution, the interior diameter of the graphite ring is dimensioned to generally correspond to the outer diameter of the glass preform, while the outer part of the graphite ring is arranged tightly along the surface of the furnace.
A drawback with the abovementioned prior art solution is that the cross-sectional shape of the glass preform is not constant. In practice, the glass preform is usually mainly circular in cross-section; however, due to practical problems during manufacture of the glass preform, the cross-sectional shape and/or the diameter may vary at some parts of the glass preform. This is problematic because in such a case the graphite ring is not able to efficiently conform to the shape of the glass preform which results in an increased gap between the glass preform and the graphite ring, or alternatively, in a situation where the surface of the glass-preform or the surface of the graphite ring may be damaged due to excessive contact forces between the surface of the glass preform and the surface of the graphite ring.
Additionally, due to the prior art sealing solution, it is not possible to simply utilize a single furnace for glass preforms of varying cross sectional dimensions. Instead, in order to avoid modifications of the furnace, a single furnace can in practice be utilized only for glass preforms with a predetermined cross-sectional dimension.